Darcy, Patrick, Count

, of a noble and ancient family in Ireland, was born in Galloway Sept. 18, 1725. His
parents, who were attached to the exiled house of Stuart,
sent him to Paris in 1739, where, being put under the
care of M. Clairault, at seventeen years of age he gave a
new solution of the problem of the curve of equal pressure
in a resisting medium. This was followed the year after
by a determination of the curve described by a heavy body,
sliding by its own weight along a moveable plane, at the
same time that the pressure of the body causes an horizontal motion in the plane. This problem had indeed been
| solved by John Bernoulli and Clairault; but, besides that
chevalier Darcy’s method was peculiar to him, we discover
throughout the work traces of that originality which is the
leading character of all his productions. The commencement of the war took him off in some measure from his
studies, and he served during several campaigns in Germany and Flanders, as captain of the regiment of Conde.
In 1746 he was appointed to accompany the troops that
were to be sent to Scotland to assist the pretender; but the
vessel in which he sailed was taken by the English, and
Darcy, whose life was forfeited by the laws of his country,
as being taken in arms against her, was saved by the humanity of the English commander. During the course of
this war, amidst all its bustle and dangers, he found leisure to contribute two memoirs to the academy. The first
contained a general principle of mechanics, that of the
preservation of the rotatory motion. Daniel Bernoulli and
Euler had found it out in 1745; but, besides that it is not
likely their works should have reached Mr. Darcy in the
midst of his campaigns, his method, which is different from
theirs, is equally original, simple, elegant, and ingenious.
This principle, which he again brought forward in 1750,
by the name of “the principle of the preservation of action,” in order to oppose it to Maupertuis’s principle of the
least action, Darcy made use of in solving the problem of
the precession of the equinoxes: here, however, he miscarried; and in general it is to be observed, that though
all principles of this kind may be used as mathematical formuloe, two of them at least must necessarily be employed
in the investigation of problems, and even these with great
caution; so that the luminous and simple principle given
by M. d’Alembert in 1742 is the only one, on account of
its being direct, which can be sufficient of itself for the sotion of problems.

Having published an “Essay on Artillery” in 1760,
containing various curious experiments on the charges of
powder, &c. and several improvements on Robins (who was not so great a mathematician as he), Darcy continued
the experiments to the last moment of his life, but has left
nothing behind him. In 1765 he published his “Memoir
on the duration of the sensation of 8i^ht,” the most ingenious of his works, and that which shews him in the best
light as an accurate and ingenious maker of experiments:
the result of these researches was, that a body may
|
souietimes pass by our eyes without being seen, or marking its
presence, otherwise than by weakening the brightness of
the object it covers; thus, in turning pieces of card painted
blue and yellow, you only perceive a continued circle of
green; thus the seven prismatic colours, rapidly turned,
produce an obscure white, which is the obscurer as the
motion is more rapid. As this duration of the sensation
increases with the brightness of the object, it would have
been interesting to know the laws, according to which the
augmentation of the duration follows the intensity of the
light, and, contrarywise, what are the gradations of the
intensity of the light of an object which motion makes continually visible; but Darcy, now obliged to trust to other
eyes than his own, was forced to relinquish this pursuit.
Darcy, always employed in comparing mathematical theory
and observation, made a particular use of this principle in
his “Memoir on Hydraulic Machines,” printed in 1754.
In this he shews how easy it is to make mistakes in looking
by experiment for the laws of such effects as are susceptible of a maximum or minimum; and indicates at the
same time, how a system of experiments may be formed,
which shall lead to the discovery of these laws. All Darcy’s works bear the character which results from the union
of genius and philosophy; but as he measured every thing
upon the largest scale, and required infinite accuracy in
experiment, neither his time, fortune, nor avocations allowed him to execute more than a very small part of what
he projected. He was amiable, spirited, lively, and a lover
of independence; a passion to which he sacrificed even in
the midst of literary society, where perhaps a little aristocracy may not be quite so dangerous.

Darcy, though estranged from it by circumstances, loved
and respected his old country: the friend and protector
of every Irishman who came to Paris, he could not help
feeling a secret pride, even in the successes of that enemy,
against whom he was so often and so honourably to himself
employed. Of his personal history, it yet remains to be
added, that in the seven years’ war he served in the regiment of Fitz-James; and in 1770 was appointed mareschal
de-camp, and the same year the academy of sciences admitted him to the rank of pensionary. In 1777 he married
a niece who was brought up under his care at Paris, and
then took the name of Count Darcy. He died two years
after this marriage, Oct. 18, 1779. Condorcet wrote his
| cloge, published in the History of the Academy, and seems
throughout anxious to do justice to his talents and character, a circumstance, which, we are told, was very highly
honourable to Condorcet, as he had been most unjustly
the continual object of Darcy’s aversion and hatred. Darcy’s essays, printed in the Memoirs of the Academy of
Sciences, are various and very ingenious, and are contained in the volumes for the years 1742, 1747, 1749, 1750,
1751, 17-52, 1753, 1754, 1758, 1759, 1760, 1765, and in
tom. I. of the “Savans Etrangers.” 1

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